1. Technical Field
The present invention relates to methods of manufacturing fluid-dynamic-pressure bearing units employed as bearing devices in applications such as spindle motors for hard-disk drives, and to motors employing such fluid-dynamic-pressure bearing units.
2. Description of the Related Art
As bearings for motors in which high rotational precision is demanded—as is the case with spindle motors employed in disk drives that drive recording disks such as hard disks, and with motors for driving the polygonal mirrors of laser printers—in order to support the shaft and sleeve letting one rotate relative to the other, fluid-dynamic-pressure bearing units that employ the fluid pressure of a lubricant such as oil intervening in between the two have variously been proposed to date.
One example of a motor that employs such fluid-dynamic-pressure bearing units is illustrated in FIG. 1. This conventional dynamic-pressure-bearing employing motor is configured with a pair of radial bearings 4, 4 in between the outer circumferential surface of a shaft 2 that forms a single component with a rotor 1, and the inner circumferential surface of a sleeve 3 through which the shaft 2 is inserted and in which it is free to rotate. In between the upper surface of a discoid thrust plate 5 that projects radially outward from the outer circumferential surface of one of the end portions of the shaft 1, and the flat surface of a step recessed into the sleeve 3, as well as in between the undersurface of the thrust plate 5 and a thrust bush 6 that closes off one of the openings in the sleeve 3, the motor is also configured with a pair of respective thrust bearings 7, 7.
Consecutive micro-gaps between the shaft 2 together with the thrust plate 5 and the sleeve 3 together with the thrust bush 6 form bearing clearances, and oil 9 as a lubricating fluid is retained continuously without interruption within these bearing clearances. (This sort of oil-retaining structure will be denoted a “full-fill structure” hereinafter.)
Herringbone grooves 41, 41 and 71, 71 composed of linked pairs of spiral striations are formed in the radial bearings 4, 4 and the thrust bearings 7, 7. In response to the rotor 1 rotating, maximum dynamic pressure is generated in the center portion of the bearings, which is where the spiral-striation joints are located. Loads acting on the rotor 1 are borne by this dynamic pressure.
In a motor of this sort, a taper seal section 8 is formed alongside a portion of the sleeve 3 at its upper end, located on the motor end axially opposite the thrust bearings 7, 7, wherein the surface tension of the oil and the atmospheric pressure balance to constitute a boundary surface. This means that the oil internal pressure within the taper seal section 8 is maintained at a pressure that is essentially equal to atmospheric pressure.
One method that has been proposed as a way of charging bearings, configured as described above, with the oil 9 as retained in between the shaft 2 with the thrust plate 5 and the sleeve 3 with the thrust bush 6 is as follows. A vacuum chamber stocked with oil is pumped down to a vacuum level, wherein a stirring device is operated to agitate and degas the oil. Then a vacuum chamber in which the bearing unit is retained is pumped down to a vacuum level, following which the oil is supplied to the vacuum chamber that retains the bearing unit, so as to put an appropriate amount of oil under a reduced-pressure environment into the bearing-unit opening, including the taper seal section 8 for the bearings. Subsequently the environment within the vacuum chamber that retains the bearing unit is brought back to atmospheric pressure, thereby exploiting the pressure difference so as to charge the bearing clearances in the fluid-dynamic-pressure bearing unit with the oil.
With oil-charging methods of the sort just described, however, air that has dissolved into the oil in the course of the oil-charging procedure, or at the stage in which after assembly as a fluid-dynamic-pressure bearing is complete the bearing is incorporated into a motor and put to work, sometimes is manifested as air bubbles.
This is thought to originate in air, slight though it may be, remaining dissolved within the oil even after having undergone the degassing process, because even with the vacuum chamber being pumped down to a vacuum level, artificially creating a perfect vacuum state is impossible. Air bubbles becoming manifest during the oil-charging procedure can hinder the smooth supply of oil from the vacuum chamber that stores oil to the vacuum chamber that holds the bearing unit, or, at the stage in which the oil has arrived inside the vacuum chamber that holds the bearing unit, can foam the oil such that the oil spouts out and sticks to the bearing unit and the vacuum-chamber interior, making it necessary to wipe the oil off, and such consequences cause a drop-off in productivity.
Moreover, if the motor is run with air bubbles within the oil mixed in as they are, eventually either of two of the following problems will arise. One affects the endurance and reliability of the motor and is a problem of the air bubbles expanding in volume-due, for example, to a rise in temperature-and causing the oil to leak out to the bearing-unit exterior. The other affects the rotational precision of the motor and is a problem of incidents of vibration or a problem of deterioration from NRRO (non-repeatable runout), due to the air bubbles coming into contact with the dynamic-pressure-generating grooves provided in the bearings.
Additional problems with the bearing oil-charging method discussed above involve a stirring propeller within the oil stored inside the vacuum chamber. Via a drive train including a shaft the propeller is linked to a drive source disposed on the exterior of the vacuum chamber. If the portion of the vacuum chamber through which the shaft passes is not hermetically sealed, then when the propeller is rotated to agitate and degas the oil, leaking of oil and dissolving of air into the oil will occur. The occurrence of such problems creates management difficulties. Furthermore, in that simply stirring the oil by the rotation of the propeller alone entails an extremely lengthy degassing operation in order to purge the oil completely of the air dissolved into it, a further concern is the consequent loss in productivity in the manufacture of fluid-dynamic-pressure bearing units.
One further concern in the manufacture of fluid-dynamic-pressure bearing units is that despite the oil having undergone a degassing process as described above, in rare instances air bubbles will be generated within the oil in installing the bearing unit into a motor and putting it to work. In such instances, given that it is unclear whether the generated air bubbles remain from or were mixed in during the oil-charging procedure, or became freshly mixed-in within the oil from the motor being driven, it is difficult to single out whether the cause is an operational shortcoming in the oil-charging procedure, or is a structural defect in, or a machining-operational shortcoming in the production of, the fluid-dynamic-pressure bearing units themselves. The consequence is that inspecting/testing to determine the cause and then finding the most appropriate way to eliminate the in-mixing of air into the oil requires an inordinate amount of time.